Merge bitcoin#17186: gui: Add placeholder text to the sign message field

7005d6a gui: Add placeholder text to the sign message field (Danny-Scott)

Pull request description:

  When using the sign message functionality I noticed the "message" field had no label or placeholder text to highlight what it's for.

  I've added the placeholder text to match the tool tip to help it be more user friendly.

ACKs for top commit:
  hebasto:
    Re-ACK 7005d6a
  fanquake:
    ACK 7005d6a

Tree-SHA512: 17fe51c134f6373d8d5f9ca98b15bd936da4e61aa5258ceb5d318575d49b43cbfde6f4c3f720eb5928206902e6ba52811ba08737a03c95224e45dabc947d9d11

Author: fanquake

src/qt/forms/signverifymessagedialog.ui

@@ -99,6 +99,9 @@
          <property name="toolTip">
           <string>Enter the message you want to sign here</string>
          </property>
+         <property name="placeholderText">
+          <string>Enter the message you want to sign here</string>
+         </property>
         </widget>
        </item>
        <item>